The present invention relates to a method of controlling the heating of an oxygen sensor fitted to the exhaust system of an internal combustion engine for the purpose of controlling air-fuel mixture air/fuel ratio, and to a system for practicing the method. More particularly, the present invention relates to such a method and device for oxygen sensor heating control which perform a time smoothing and time delaying of the power dissipated in a resistive electrical heater element of the oxygen sensor, so as to ensure that the oxygen sensor is kept at a proper temperature even when engine operational parameters change at a high rate.
It is known to fit an oxygen sensor to the exhaust system of an internal combustion engine. Such an oxygen sensor typically comprises a solid electrolyte or semiconductor, and varies a generated current or resistance in response to the concentration of oxygen in the exhaust gases of the engine. This electrical signal is fed to a control device which controls the amount of fuel provided to the engine in relation to the amount of air sucked thereinto, and is used for controlling the air/fuel ratio of the air-fuel mixture supplied to the engine by a feedback process. Various such forms of control device, which practice various methods of air-fuel mixture ratio control, are per se known.
The output of the sensor element of such an oxygen sensor varies with temperature, and, particularly when the air/fuel ratio is weak and is in the range of 17 to 25, in order for the sensor element to accurately measure the oxygen concentration, said sensor element must be maintained at a temperature higher than a certain critical minimum active temperature. This maintenance of the temperature of the sensor element can be done by using a heater, and oxygen sensors with sensor element heaters have already been proposed, along with methods for operation of such heaters; for example in Japanese Patent Application No. 53-78476, which has been published as Japanese Patent Application No. 54-13396. Further, in Japanese Patent Application No. 53-83120, which has been published as Japanese Patent Publication No. 54-21393, there has been proposed a method and a system for control of the electrical power supplied to such an oxygen sensor element heater, in which the power is varied as a function of intake manifold pressure, of throttle opening, and of engine revolution speed, so as to ensure that the oxygen sensor element is kept at a temperature no lower than its minimum active temperature.
The sensor element of such an oxygen sensor fitted to an exhaust system is of course heated up by the exhaust gases in the exhaust system, so the effect of a heater for the sensor element must be controlled to take account of the temperature of these exhaust gases. Now, in an internal combustion engine which is controlled by a throttle valve, the exhaust temperature is largely determined by the amount of air-fuel mixture supplied per engine piston stroke and by engine revolution speed, and if the air/fuel ratio of the air-fuel mixture is constant the amount of such mixture supplied is proportional to the rate of intake air flow. Therefore, in the above mentioned patent applications, the above are used as parameters, and the supply of electricity to the sensor element heater is varied depending on the engine load and the engine revolution speed. Thus, the exhaust temperature is considered to depend on the engine intake flow and engine revolution speed, and the values are determined experimentally in advance with reasonable accuracy. This method and system are adequate to keep the temperature of the sensor element of the oxygen sensor reasonably constant regardless of engine operational conditions, provided however that these engine operational condition do not change too abruptly.
However, when the operating parameters of the engine change abruptly, this method and system do not provide satisfactory operation. This is because the temperature of the exhaust gases does not react instantaneously to changes of the engine operating parameters, such as throttle opening, engine revolution speed, engine load, intake system pressure, intake system flow rate, and so on, but instead reacts with a certain characteristic time lag, which may be termed an inertial smoothing effect. If therefore as suggested above the operation of the oxygen sensor heater is controlled as a strict function of such engine operational parameters, then when the values of the relevant parameters change the amount of power that is being supplied to the oxygen sensor heater will accordingly be changed substantially simultaneously, and this will not be in accordance with the actual ongoing temperature of the exhaust gases. For example, if from a low load engine operating condition the accelerator pedal of the vehicle is stepped upon, which will in due course after the aforesaid time lag cause the temperature of the exhaust gases to rise, then according to such a system as above outlined the amount of power supplied to the oxygen sensor heater will immediately be reduced in anticipation of the aforesaid exhaust gas temperature rise, thus allowing the temperature of the oxygen sensor undesirably to drop, before the warming up thereof by the increase in temperature of the exhaust gases can take place. This will cause a temporary drop in the temperature of the oxygen sensor heater, which can be very troublesome, and can lead to poor operation of the air/fuel ratio control system, and to high levels of emission of pollutants in the exhaust gases of the engine as well as to poor engine operation and drivability. Likewise and conversely, if from a high load engine operating condition the accelerator pedal of the vehicle is released from being depressed, which will in due course after the aforesaid time lag cause the temperature of the exhaust gases to drop, then according to such a system as above outlined the amount of power supplied to the oxygen sensor heater will immediately be increased in anticipation of the aforesaid exhaust gas temperature drop, thus allowing the temperature of the oxygen sensor undesirably to rise, before the cooling down thereof by the drop in temperature of the exhaust gases can take place. This will cause a temporary rise in the temperature of the oxygen sensor heater, which can lead to overheating thereof and damage thereto. This phenomenon can also occur when the engine of the vehicle is stopped while the ignition switch thereof is left in the on condition (so called "hot soaking").
Both of these troublesome operational modes of the prior art are illustrated in FIGS. 8 and 9 of the accompanying drawings, which are mutually coordinated graphs showing, against time, various quantities relating both to the operation of an engine incorporating a prior art oxygen sensor heating system and to the operation of an engine incorporating an oxygen sensor heating system according to the present invention. In FIG. 8, the depression from idling position, and the restoration thereof, of the throttle pedal of the vehicle incorporating the engine are shown by the dip in the uppermost solid line, and the subsequent rise thereof, and the lines denoted by Pm, Ne, and Te represent respectively pressure in the engine intake system, engine revolution speed, and exhaust gas temperature. Thus, it is seen that the time points of the rise in the exhaust gas temperature and of the subsequent fall thereof are significantly delayed after the time points of the depression of the accelerator pedal and of the subsequent release thereof. Correspondingly, in FIG. 9, in the case of the prior art described above the time variation of the power supplied to the heater element is shown by the dashed line, and this is advanced in time with relation to the variation of the exhaust gas temperature Ts shown in FIG. 8, so that the temperature of the sensor element of the oxygen sensor, which is shown by the double dotted line, is quite high, first being a temperature drop when the accelerator pedal is depressed and then a temperature rise when the accelerator pedal is released again.